I had hit 40 with rather too heavy a thud. My daughter had taken to wearing my clothes and, worse, looking better in them than I did. Four grey hairs had landed on my head like aliens from another planet and my favourite jeans had mutated into an instrument of torture around my waist.

I decided I had to get fit, but since I am anaemic where iron-like determination is concerned, it was imperative to find a form of exercise that was enjoyable. That ruled out anything like jogging (too boring) or squash (too frightening).

Yoga seemed worth a try, so I turned up for an evening class. The teacher was a walking example of its benefits. She was slim, unlined and spoke in placid, unhurried tones.

Yoga is definitely for those who fancy a gentle introduction to exercise and yearn for a weekly hour of calm and tranquillity. But I am so unused to calm and tranquillity that I fell asleep in the first lesson. After the second lesson and another long snooze, I realised that I didn't have the willpower to stay awake, let alone to wrap a leg around my neck.

Perhaps aerobics was the answer: no danger of falling asleep, and look what it did for Jane Fonda.

I can confidently report that if you are young, like loud music and want to push your body till it hurts, you will love this. My teacher, a gleaming blonde with muscles straight out of a biology textbook, yelled instructions above the music, which my feet vainly tried to follow. I felt like an elephant trying to dance Swan Lake. My advice to anyone with such poor physical co-ordination is: try something else.

Swimming is an excellent hobby because it exercises every muscle in the body. If you can get to a pool first thing in the morning, you will probably enjoy the peaceful, rhythmic to-ing and fro-ing as you swim your lengths. If, like me, you can only go in the evening or during your lunch hour, you may well decide that the struggle to avoid all the other bodies in the water is not worth the effort. It is like trying to walk backwards in the rush hour at Charing Cross station.

'Try Pulse-rate,' suggested my son, Pulse-rate being the name of our sports centre's gym. My son had been going for more than a year and his physique had definitely improved. I wasn't quite sure that broad shoulders and huge biceps were at the top of my agenda, but my son insisted.

At the 'induction lesson' I was greeted by a young amazon who introduced me to Sue, 'another beginner'. Sue looked like she had been born in a leotard.

The amazon questioned us about our lifestyles and our reasons for going to Pulse-rate. Sue wanted to improve her overall fitness. I muttered something about flat tummies and tight bottoms.

I took one look at all that gleaming machinery and wondered if I should excuse myself for a suddenly imagined doctor's appointment. We were shown how to use the jogging machine, the rowing machine, the cycling machine. We were taught about leg extensions, lateral pulldowns and triceps pushdowns. I emerged exhausted but elated. Every part of my body had been stretched - and it seemed to like it.

Six months on, I am still enthusiastic and I honestly feel I am embarking on my middle years a good deal fitter than I was in my youth. I have also discovered that physical exercise is the best remedy for depression and apathy. Once you take control of your body, you feel more confident about tackling other areas of your life.

I wish I could tell you that I now have the physique of Jane Fonda. I don't - yet. But anyway, I hear she has now resorted to plastic surgery. Shame on you, Jane. You just need the willpower.